This disclosure relates generally to pneumatic or hydraulic systems that produce mechanical energy, or work. In particular, the disclosure relates to a pneumatic, or hydraulic, system with a regulator formed by a motor that enables pressure reduction of a fluid and produces mechanical energy that may be used to drive a compressor, or pump.
Many conventional pneumatic systems include a motor to which compressed air is delivered. In turn, the motor produces mechanical energy, or work, that may be used, for example, to power a hand-held tool. To control the pressure of compressed air delivered to the motor, the pneumatic system may also include a regulator positioned upstream of the motor. Compressed air is delivered through the regulator, causing a reduction in the pressure of the compressed air such that the pressure of compressed air delivered to the motor is at a desired level.
FIG. 1 illustrates a conventional pneumatic system. Pneumatic system 10 includes a source of compressed air 15, a motor 20, a regulator 25 disposed upstream of motor 20, and an actuatable device 30 coupled to motor 20. Compressed air is delivered from source 15 through regulator 25 to motor 20. Assuming isentropic flow through motor 20 and potential and kinetic energy are negligible, the amount of mechanical energy, or work, Wprod,20 produced by motor 20 may be calculated using the First Law of Thermodynamics, which under the assumed conditions, simplifies to:Wprod,20={dot over (m)}*(h3−h2)  (1)where h2 is the enthalpy of compressed air entering motor 20; h3 is the enthalpy of compressed air exiting motor 20; and {dot over (m)} is the flow rate of compressed air through system 10. In pneumatic system 10 illustrated by FIG. 1, equation (1), and equation (2), included below, the subscript “3” denotes a property of compressed air exiting motor 20. Thus, h3 represents the enthalpy of compressed air exhausted motor 20. The subscript “2” denotes a property of compressed air exiting regulator 25 and entering motor 20. The subscript “1” denotes a property of compressed air supplied by source 15 to regulator 25.
Work Wprod,20 produced by motor 20 is then used to actuate device 30. As previously mentioned, device 30 may be a hand-held tool powered by motor 20. Alternatively, device 30 may be an object that is lifted or moved, such as a hydraulic cylinder in a blowout preventer. Moreover, device 30 may be a generator. In short, device 30 may be any apparatus that is actuated by mechanical energy.
As compressed air flows through pneumatic system 10 and work Wprod,20 is produced by motor 20, energy is removed from the compressed air as it passes through regulator 25 to enable pressure reduction of the compressed air. Because the energy removed is not utilized, it may be considered wasted. The amount of energy wasted Wwasted may be calculated using an Exergy Rate Balance Equation, which under the assumed conditions, simplifies to:Wwasted={dot over (m)}*(h2−h1)  (2)Depending on the design configuration of regulator 25, the energy wasted Wwasted as the compressed air passes through regulator 25 maybe significant, and particularly so when compared to the amount of work produced Wprod,20 by motor 20.
For exemplary purposes, the following conditions are assumed: the pressure P1 and temperature T1 of compressed air delivered from source 15 to regulator 25 are 20 MPa and 300° K., respectively; the compressed air flowrate {dot over (m)} through system 10 is 1 kg/second; a pressure reduction of 19 MPa occurs through regulator 25; and compressed air exits motor 20 with a pressure P3 of 0.2 MPa. Based on these conditions, the state of compressed air entering regulator 25 may be fully defined: pressure P1=20 MPa (given), temperature T1=300° K. (given), enthalpy h1=267.80 kJ/kg, and entropy s1=5.25 kJ/kg. Next, the state of compressed air exiting regulator 25 and entering motor 20 may be fully defined. Given an assumed 19 MPa pressure reduction through regulator 25 and isentropic flow through regulator 25, the properties of compressed air exiting regulator 25 and entering motor 20 are: P2=P119 MPa, or 1 MPa, entropy s2=s1=5.25 kJ/kg, enthalpy h2=110.20 kJ/kg, and temperature T2=123.75° K. Lastly, the state of compressed air exiting motor 20 may be defined. Based on the assumed pressure P3=0.2 MPa at the exit of motor 20 and isentropic flow through motor 20, the properties of compressed air exiting motor 20 are: pressure P3=0.2 MPa (given), entropy s3=s2=5.25 kJ/kg, enthalpy h3=69.32 kJ/kg, and temperature T3=87.75° K.
Having fully defined the state of compressed air entering regulator 25, exiting regulator 25 (also entering motor 20), and exiting motor 20, the work produced Wprod,20 by motor 20 is estimated to be 55 hp using equation (1). Also, the work wasted Wwasted as compressed air passes through regulator 25 is estimated to be 211 hp using equation (2). As demonstrated, a significant amount of energy is wasted during pressure reduction of the compressed air as it passes through regulator 25.
Accordingly, apparatus or systems that enable use of the energy removed from the compressed air during pressure reduction are desirable.